1. Field of the Invention
The present invention relates to a photodetecting device, a radiation detecting device, and a radiation imaging system having the photodetecting device and the radiation detecting device. More specifically, the present invention relates to a technique that is suitably used for a radiation detecting device for detecting radiation, such as x-rays or γ-rays, and is applied to a medical image diagnosing apparatus, a nondestructive inspecting apparatus, an analyzing apparatus that uses radiation, or the like.
2. Related Background Art
In recent years, as a radiation detecting device that detects radiation, such as x-rays or γ-rays, there has been used a so-called indirect-type radiation detecting device that converts radiation into visible light and detects the visible light with using a photoelectric conversion element that uses an amorphous silicon thin film. As a main reason why the radiation detecting device of this type is commercialized, there is cited a fact that the advancement of a liquid crystal technology, whose core is amorphous silicon having photoconductivity, makes it possible to increase the areas of TFTs (Thin Film Transistors) and photosensors. Another main reason is that the combination with GOS phosphor, CsI phosphor, or the like, which have conventionally been used, makes it possible to produce, with stability, a radiation detecting device that has a large screen and a high degree of reliability.
As a typical example of the radiation detecting device of this type, there has conventionally been used a radiation detecting device in which the phosphor described above is combined with a photosensor array in which there are arranged a plurality of pixels that each have an MIS-TFT structure constructed of a switching TFT and an MIS-type photoelectric conversion element proposed by the inventors of the present invention. The feature of the photosensor array in this example is that it is possible to manufacture the switching TFT and MIS-type photoelectric conversion element described above in the same layer through the same process. As a result, there is obtained an advantage that it becomes possible to produce the photosensor array with stability and at low cost.
Meanwhile, there are also proposed various other photosensor arrays, such as a photosensor array in which there are arranged a plurality of pixels that each have a PIN-TFT structure obtained by combining a PIN-type photoelectric conversion element with a switching TFT or a photosensor array in which there are arranged a plurality of pixels that each have a PIN-PIN structure obtained by using a PIN-type diode for a switch element. As a matter of principle, these photosensor arrays generally use a common driving method with which radiation is converted into visible light using a phosphor, the visible light is stored as accumulated charges by a photoelectric conversion element, and the charges are read in succession by a switch element.
FIG. 14 shows a schematic equivalent circuit of a photosensor array that is used for a general radiation detecting device. In this drawing, to simplify the explanation, there is used, as an example, a photosensor array constructed of a 3- by 3-pixel matrix that has nine pixels in total.
Each pixel is constructed of one photoelectric conversion element Sij (i, j=1 to 3), one switching TFT Tij (i, j=1 to 3), and the like. In this example, as to the photoelectric conversion element Sij, there is not drawn a distinction between the aforementioned MIS type and PIN type in this drawing. Also, in FIG. 14, reference symbol Vsn (n=1 to 3) represents bias wiring of a photoelectric conversion element that is connected to a bias power source B. Reference symbol Vgn (n=1 to 3) represents the gate wiring of a switching TFT and reference symbol Sign (n=1 to 3) represents a signal line. The signal output from each photoelectric conversion element Sij is accumulated in the photoelectric conversion element Sij itself. Then, according to the output signal from a driving circuit D, the switching TFTs Tij are turned on in succession and currents corresponding to the accumulated charges accumulated in the photoelectric conversion element Sij itself flow to the signal line Sign (n=1 to 3). The signal read in this manner is inputted into a signal processing circuit A, which then amplifies, A/D-converts and outputs the signal as an output signal.
FIG. 15 is a schematic plan view of one pixel of the MIS-TFT structure. FIG. 15 is a diagram taken from a side on which there are arranged a source-drain electrode and the like under a condition where a phosphor is not yet bonded. Each solid line represents a portion that can be viewed from a side on which a signal line is arranged, while each dotted line indicates a portion that cannot be viewed therefrom.
In FIG. 15, one pixel includes: a photoelectric conversion element constructed of a sensor section 50, a lower electrode 3, and bias wiring 8; a switching TFT constructed of a gate electrode 4, a source-drain electrode 9, and the like; gate wiring 2 that is the gate wiring of the switching TFT; a signal line 10 that is a signal line for transferring an electronic signal converted by the photoelectric conversion element; and a contact hole 12 for electrically connecting the source-drain electrode 9 of the switching TFT to the lower electrode of the photoelectric conversion element.
FIG. 16 is a schematic cross-sectional view of the above described one pixel. FIG. 16 is a drawing in which a cross-sectional view of each device taken in an arbitrary direction is arranged to explain the layer construction of each device, such as the gate wiring, the photoelectric conversion element, the switching TFT, and the signal line shown in FIG. 15. Note that the order in which each layer is constructed is the same as that in FIG. 15.
In FIG. 16, reference numeral 1 denotes a glass substrate, numeral 3 the lower electrode of the photoelectric conversion element, numeral 8 the bias wiring of the photoelectric conversion element, numeral 2 the gate wiring of the switching TFT, numeral 4 the gate electrode of the switching TFT, and numeral 9 the source-drain electrode of the switching TFT.
A layer, from which the sensor section of the photoelectric conversion element and the source-drain electrode of the switching TFT are removed, and the lower portion of the signal line are formed in the same layer. Here, reference numeral 5 denotes an insulating film, numeral 6 an amorphous silicon film functioning as an active layer, and numeral 7 an ohmic contact layer. Reference numeral 10 denotes a signal line connected to the switching TFT. Also, reference numeral 100 denotes a protecting film, numeral 101 a bonding layer, and numeral 102 a phosphor layer. There is obtained a construction where incident radiation is made incident from a side on which the phosphor layer exists in the drawing.
Currently, there exists an increasing demand for achieving higher sensitivity of the radiation detecting device of this type to realize the reduction of the amount of radiation. Also, it is hoped to achieve higher definition to realize the higher quality of an image. Under these circumstances, there have been made various developments, such as the improvement of the luminous efficiency of a phosphor, the improvement of the light collection efficiency in a photosensor array, and the improvement of a photoelectric conversion element itself, which is to say a photoelectric conversion efficiency.
In general, in the case where high image quality is achieved, it is first required to obtain a finer pixel pitch. However, if there is obtained such a finer pixel pitch without intricacy, this inversely causes the reduction of sensitivity, which means that it is impossible to realize the finer pixel pitch without complication.
The reason of this is that an effective pixel region is not reduced in accordance with the enhancement of definition but the effective pixel region is required to have the same or a larger area.
That is, it is required to accelerate the driving speed of a switching TFT, a signal processing speed, and the like in accordance with the increase of the number of pixels and it is also required to further reduce the resistances of the driving wiring of the switching TFT, the signal line, and the like. In some cases, it is also required to increase the size of the switching TFT to reduce the on-resistance of the switching TFT or for other purposes.
That is, the ratio of the width of each wiring to the area of a pixel is increased in accordance with the enhancement of definition and the area occupied by the switching TFT does not greatly change, so that the effective aperture ratio of the photoelectric conversion element occupying an area of a pixel generally tends to be reduced in accordance with the reduction of a pixel pitch.
As described above, in the case where higher definition is realized while maintaining a large area, there is reduced the effective aperture ratio of a pixel. As a result, to obtain certain image quality, it is required to increase the amount of radiation, which is not acceptable in a medical field when attentions are paid to the effect on a human body.
FIG. 12 is a plan view illustrating the effective aperture ratio of one pixel. In FIG. 12, reference symbol P represents a pixel pitch, symbol Vg the gate wiring of a switching TFT, symbol Sig a signal line, and symbol S a sensor region of a photoelectric conversion element. When the width of the gate wiring Vg is referred to as Wg, the width of the signal line Sig is referred to as Ws, a clearance between the gate wiring Vg and the sensor region S of the photoelectric conversion element is referred to as Lg, and a clearance with the signal line 10 is referred to as Ls, it is possible to roughly calculate an effective aperture ratio Ap using the following equation.Ap=(P−Wg−2Lg)×(P−Ws−2Ls)/P2
In reality, however, there exists a switching TFT, so that the actual effective aperture ratio takes a value that is smaller than the effective aperture ratio Ap calculated using this equation. However, to simplify the explanation, the area of the switching TFT is not taken into consideration in this description.
Next, as an example, FIG. 13 shows how an effective aperture ratio changes in accordance with the increase of a pixel pitch under a condition where the width Wg of the gate wiring Vg is set at 10 μm, the width Ws of the signal line Sig is set at 8 μm, and the clearances Lg and Ls of the signal line Sig and the gate wiring Vg are each set at 4 μm.
Here, as to the clearance Lg, it is required that the gate wiring, the gate electrode, and the lower electrode of the photoelectric conversion element are electrically insulated and these construction elements are arranged in the same layer. Therefore, in reality, the clearance Lg exists as a space of about 4 μm due to the limited performance of a manufacturing apparatus and the like.
Also, as to the clearance Ls, the MIS construction of the photoelectric conversion element and the MIS construction of the lower portion of the signal line are electrically separated from each other. Therefore, in reality, the clearance Ls exists as a space of about 4 μm, similar to the clearance Lg.
As is apparent from this drawing, it can be confirmed that the effective aperture ratio Ap is sharply reduced in accordance with the reduction of the pixel pitch P, with the sharp reduction starting at a point where the pixel pitch P becomes about 70 or 80 μm. That is, under present circumstances, it is difficult to realize higher definition of about 70 or 80 μm because such higher definition causes the great reduction of sensitivity.
On the other hand, in the case of a pixel pitch of about 150 μm to 200 μm that has conventionally been used in general cases; even though the effective aperture ratio Ap is improved, it cannot be said that this pixel pitch is satisfactory enough in view of sensitivity at the present stage. That is, as described above, there are imposed limitations concerning the width of each wiring, the size of a switching TFT, and the like, so that it is not expected that higher sensitivity is realized under present circumstances, due to a limitation in improving the effective aperture ratio.
In view of the problems described above, the object of the present invention is to make it possible to improve the effective aperture ratio of a pixel, achieve higher sensitivity, and realize higher definition without causing the degradation of performance of gate wiring, a signal line, and a switching TFT that are indispensable in constructing a pixel.